Digital and electronic equipment is frequently subject to malfunction in the presence of radio frequency interference in the power line supplying it. Such equipment is not only susceptible to such interference; it is also often highly productive of it and thus may feed out interference through the power lines to other susceptible apparatus. Filters must therefore be interposed between the power source and the equipment to screen out such interference from the equipment and also interference generated by the equipment from returning to the power line.
This interference is analyzable into two sorts of distortion of the power current wave form: common mode interference where identical wave forms in reference to ground appear on the power lines connected to the equipment, and differential mode interference which appears as a voltage difference between the two power lines.
Circuitry exists to filter out radio frequency interference, but for optimum effectiveness and cost, it has been found necessary to treat the two sorts of interference independently. One configuration of filter elements with certain value relationships is optimal for common mode noise e.g., low value capacitors and high value inductance, whereas a different configuration with different value relationships, low value inductance and high value capacitance, is optimal for differential mode noise. Such filter elements can be combined in a single package, but the distinction of function remains. It is important, therefore, that the two noises be separately and accurately assessed, both for filter design and for monitoring filter effectiveness.
One past approach to the isolation of the two modes of noise and the quantitative expression thereof has involved power splitter/combiners. The idea of that approach, as in the present invention, is to bring about a cancellation of one noise mode while augmenting the other. To restate the difference between the two noise modes: noise in each of a pair of power lines is analyzable into a component which is of identical wave form or equal instantaneous voltage in reference to ground to that in the other power line; and into another component which is opposite in wave form or instantaneous voltage to that in the other power line. A zero degree splitter/combiner is used in its combining function to sample the r-f voltages present in the two lines and combine these signals. This, in theory, should result in cancellation of the differential mode noise and a doubling of the common mode noise which then can be projected on a spectrum analyzer for example. For the detection of differential mode noise, a 180 degree splitter/combiner is employed which, again in theory, should result in cancellation of the common mode noise and a doubling of the differential mode noise.
One difficulty with the use of splitter/combiners for this purpose lies in the fact that the 180-degree splitter/combiner relies on passive elements for its operation, and phase displacements do not reach the theoretical (and necessary) full 180-degrees for cancellation of the unwanted noise component. Similarly, a precise zero-degree phase shift over a wide band of frequencies is not achieved by a zero-degree splitter/combiner.